The present invention relates to a method of preparing sodium monosulfide by means of reacting a sodium polysulfide with sodium under a protective gas.
Sodium monosulfide, Na.sub.2 S, is an important reagent for introducing sulfur into organic molecules. Several methods are known according to which this compound can be prepared. It can be obtained, for example, by means of reacting sodium salts with hydrogen sulfide in aqueous or alcoholic solutions or by means of the reduction of sodium sulfate and carbon or hydrogen. A problem with such known methods is the fact that products contaminated with reactants always accumulate which must be separated from the impurities by being dissolved in suitable solvents and by filtering.
Since the elements sodium and sulfur react extremely vigorously with one another (enthalpy of formation for Na.sub.2 S:.DELTA.H.sub.B =-389.1 kJ/mole), a direct method of preparation from the elements which are commercially available in great purity could not feasibly be used for industrial purposes in the past.
Moisture-free Na.sub.2 S in sufficiently pure form was only obtainable by means of a process involving dewatering the hydrate Na.sub.2 S.9H.sub.2 O under an atmosphere of hydrogen. This hydrate had to be prepared by means of reacting sodium hydrogen sulfide with NaOH in a polar solvent (Kirk-Othmer, 3d edition (1982), vol. 18, pp. 793-847, especially pp. 803 and 809).
The prior art, namely, DE-PS 34 36 698 discloses a method of preparing sodium polysulfides from the elements sodium and sulfur in which the sodium and the sulfur under protective gas in a stoichiometric proportion corresponding to the desired polysulfide are charged alternately into a melt of a polysulfide placed in a reaction receiver means under vigorous agitation. The portions are measured in such a manner that during the charging of sodium the reaction mixture remains in the state of an agitatible suspension and during the charging of the sulfur the latter is allowed to react completely in each instance to form a polysulfide of a higher sulfur content.
German patent application P 38 31 737.0 describes a method of preparing low sodium polysulfides from higher sodium polysulfides and sodium in which the higher sodium polysulfide is placed in a reaction receiver vessel under a protective gas in molten form and the sodium is charged into the melt in a stoichiometric proportion corresponding to the desired polysulfide product. Vigorous agitation is used in the process. Amounts of reactants used are such that the reaction mixture remains in the state of an agitatible suspension. The agitation is continued until the formation of the product has been completed.
It would also be basically possible according to these two methods to recover sodium monosulfide by providing the required stoichiometric proportion for the combined together sulfur and the sodium. However, since the known methods require a melt as reaction medium which consists at least at the final stage of the reaction process of Na.sub.2 S alone, an economically feasible reactor vessel material would hardly be available as a consequence of having to contain the very high melting point Na.sub.2 S compound (1180.degree.-1200.degree. C.) and on account of the strong chemical aggressivity of the melt.